1. Field of the Invention
The present invention relates to a semiconductor device for evaluating the surface and the interface of a semiconductor and a method of evaluating the surface and the interface of a semiconductor by the use of the semiconductor device.
2. Description of the Related Art
In a semiconductor device such as a field effect transistor (hereinafter referred to as FET), the trap of the surface and the interface of a semiconductor (hereinafter referred to as a surface state/interface trap) and the presence of ions placed on the surface cause a problem of a trap response such as a frequency dispersion. The surface state/interface trap and the presence of ions are also important factors for determining basic properties such as a breakdown voltage of the semiconductor device. Frequency dispersion herein means a phenomenon in which a current is changed due to the frequency of an alternating voltage applied to the semiconductor device. However, the actual surface state/interface trap is changed in accordance with the method of fabricating the semiconductor device and in accordance with the material being in contact with the interface. During fabricating the semiconductor device, it is thus difficult to strictly control the energy level of the trap and the concentration of the ions placed on the surface.
The ions placed on the surface during and after fabricating the semiconductor device acts as an interface charge, so that it has an influence on the operation of a FET. The ions are thus difficult to completely remove. It is therefore necessary to evaluate the fabricated semiconductor device and to obtain information about the surface state/interface trap and the ions placed on the surface.
As methods of measuring and evaluating the surface state/interface trap, the method of measuring the frequency dispersion by the use of the FET and the measuring method using DLTS (Deep Level Transient Spectroscopy)/ICTS (Isothermal Capacitance Transient Spectroscopy) have been heretofore employed. However, it is difficult to electrically measure the concentration of the ions placed on the surface.
Additionally, during a measurement of the frequency dispersion, a voltage, resulting from the alternating voltage superposed on a direct-current voltage, is applied to a drain electrode and a gate electrode of the FET. In this way, a conductance of the FET and a phase difference between the current and the voltage are detected. A change due to the frequency is then checked, whereby the presence of the trap is judged. On the other hand, using the DLTS/ICTS, method a current transient response is observed when the voltage of the drain electrode and the gate electrode of the FET is changed in a pulse manner, whereby the presence of the trap is observed.
These conventional measuring methods make use of the fact that the change in a charge state of the surface state/interface trap due to an alternating signal and pulse input causes the change in a potential distribution so as to thereby modulate the current. The measurement is performed by changing a temperature, whereby the energy level of the trap and a capture cross section can be also calculated.
However, in these methods, confirmation of the presence of the surface state/interface trap depends on a detection of a time constant. Thus, if the time constant is within a time scale in which the measurement is performed or within a frequency range, the presence of the trap can be confirmed. However, when the energy level of trap is shallow or when the response is made by the use of a time constant different from the time constant calculated from the energy level of the trap and the capture cross section, the presence of the surface state/interface trap cannot be detected or can be falsely detected.
Since the FET is often used for the measurement, the change in the voltage applied to the drain electrode and the gate electrode causes the change of the surface state/interface trap in an unintended place and the change in the charge state of the trap in a substrate. Consequently, a measurement result includes even these trap responses. Since it is difficult to separate the response alone from the target surface state/interface trap, it is difficult to specify the place of the surface state/interface trap to be observed.
Although whether the trap concentration which is one of parameters associated with a recombination rate is higher or lower can be determined from a comparison of two results, the concentration value is difficult to calculate. Thus, since the measurement result is observed only in the form of a current variation and the conductance change, it is not possible to judge the recombination rate through the surface state/interface trap.